IMHO, for sumo-bots mass and traction (grip on the playing surface) is important.
Servos (modified for continuous rotation) have more mass than motors BUT motors have a better selection of wheels/tires.
- wheel/tire selection
- number of I/O pins needed
+ many wheel/tires to choose from (adds slightly more mass)
+ modify torque by changing gear sets (adds slightly more mass)
+ one I/O pin for motor driver
+/- you need a motor driver (adds slightly more mass)
- less mass
Everything else depends on your programming skill.
I really like the design of this sumo-bot:http://www.huv.com/miniSumo/seeker2/index.html